A shock absorber of this type is illustrated by way of example in FIG. 1. This shock absorber of the prior art has a housing with an inner tube arranged therein, a piston rod immersing into the inner tube, and arranged on the end of the piston rod immersing into the inner tube is a piston which divides the interior of the inner tube into a lower chamber and an upper chamber. In this respect, arranged on the piston is a valve arrangement by which a working medium received in the inner tube can flow out of the lower chamber into the upper chamber and vice versa as the piston travels in the inner tube. Arranged at the upper end of the inner tube is a second valve arrangement by which the working medium received in the inner tube can only flow out of the upper chamber into the interior of the housing acting as a tank for the working medium, as the piston travels in the inner tube. Finally, at its lower end, the inner tube has a third valve arrangement by which the working medium received in the interior of the housing acting as a tank can only flow into the lower chamber of the inner tube as the piston travels in the inner tube.
Prior art shock absorbers of this type are usually known as twin tube shock absorbers. Such twin tube shock absorbers are used as passive shock absorbers, on the one hand as non-adjustable shock absorbers and, on the other, as adjustable shock absorbers.
In the case of a non-adjustable twin tube shock absorber, the characteristics of the shock absorber or of the damping respectively are firmly defined, so that the flow resistance of the working medium is fixed by the valve arrangements used in the shock absorber.
Adjustable twin tube shock absorbers are used when the work to be performed by the shock absorber is to be adapted to corresponding states of a vehicle in which an adjustable twin tube shock absorber of this type is fitted, or respectively to sprung, different-sized masses. In the case of passive hydraulically adjustable twin tube shock absorbers of this type, this is achieved in that the flow resistance can be changed by a variable valve bore in the valve arrangement between the upper chamber and the lower chamber of the inner tube of the shock absorber, which valve arrangement is arranged on the piston.
A further feature of passive, non-adjustable shock absorbers is that the quantity of oil exchanged at a defined working stroke is always the same. The flow resistance depends only on the speed of the piston inside the inner tube.
In the case of passive, adjustable shock absorbers, likewise at an identically defined stroke, the flow resistance is influenced by the alteration of the valve cross section of the valve arrangement in the piston, in addition to the speed of the piston. The exchanged quantity of oil is also the same at a defined stroke.
In the following, the mode of operation of a twin tube shock absorber of this type according to the prior art will be briefly described.
When the shock absorber moves in, the piston in the inner tube moves downwards so that the lower chamber of the inner tube decreases in volume, while the upper chamber of the inner tube increases in volume. During this moving-in action of the shock absorber, a particular quantity of oil, serving here as the working medium, is guided by the valve arrangement of the piston from the lower chamber into the upper chamber and, as a result, a defined flow resistance is produced via the valve arrangement of the piston by the speed of the piston. The quantity of oil in the upper chamber of the inner tube, which oil also displaced by the immersion of the piston rod, is diverted into the housing acting as a tank for the working medium or oil respectively by the valve arrangement arranged at the upper end of the inner tube.
When the moving-out action of the shock absorber is initiated, the piston moves upwards inside the inner tube due to a reduction in volume in the upper chamber and an increase in volume in the lower chamber, the quantity of oil or working medium respectively to be exchanged being guided from the upper chamber into the inward of the housing acting as a tank for the working medium respectively into the oil sump there by the valve arrangement at the upper end of the inner tube. The difference in quantity of oil or working medium respectively required by the moving-out motion of the piston rod is subsequently suctioned up by the valve arrangement arranged at the base of the housing acting as a tank for the working medium respectively into the oil sump there.
As just illustrated, the working medium or oil respectively moves in a circuit in the shock absorber.
Since a twin tube shock absorber of this type has to be prefabricated accordingly for its passively adjustable or passively non-adjustable use, it is not possible to use a shock absorber provided on one occasion for a different application purpose.